Anti-whaling activists today accused Japanese whalers of ramming and sinking one of their boats as international tension over Japan's annual "scientific" culls in Antarctic waters grew.

Japanese fisheries officials said the Ady Gil, a high-speed boat belonging to the Sea Shepherd conservation group, had been hit accidentally as it attempted to confront the Shonan Maru No 2, a whaling vessel.

Sea Shepherd said its stricken boat, a trimaran made of fibreglass, had been "sliced in half" after being rammed near Commonwealth Bay.

The boat's six crew members were rescued and taken aboard another of the group's boats.

One crewman, a New Zealander, is thought to have suffered two cracked ribs after being thrown to the deck in the collision.

Another remained aboard the Ady Gil today, attempting to salvage as much equipment as possible from the sinking ship.

Sea Shepherd's founder, Paul Watson, accused the whalers of deliberately ploughing into the front of the boat. "It was a vicious attack," he told the Guardian by satellite phone from the group's mother ship, the Steve Irwin.

"The Shonan Maru turned and deliberately struck the Ady Gil, cutting it in half. The captain [of the Ady Gil] was trying to reverse to get out of its way when it happened.

"If he hadn't done so, the Shonan Maru could have struck another part of the boat and killed someone."

In a statement, the group said the Ady Gil was missing about 10ft (three metres) of its bow, adding that there was little chance of salvaging the wreckage.

Watson said the loss of the $1.8m craft, which was paid for by a California businessman, would not deter Sea Shepherd from pursuing the Japanese fleet, which this year plans to kill 935 minke whales and 50 fin whales.

"We will be back chasing them, cutting their quota and their profits," he said. "We still have another two months left of this."

His claims about the collision brought an angry response from Japanese officials, who accused Sea Shepherd of engaging in "extremely dangerous" behaviour.

The fisheries agency said the collision happened when the Ady Gil ignored warnings and suddenly slowed down as it crossed front of the Shonan Maru.

"These acts of sabotage that threaten our country's whaling ships and crew were extremely dangerous," the agency said in a statement. "It is totally unforgivable."

The Shonan Maru emerged unscathed and none of the crew members were injured, the agency added.

The Institute of Cetacean Research, a government-funded body which organises the culls, said the Ady Gil had come "within collision distance" of a whaler's bow and had dangled a rope, which could have entangled the whaling ship's rudder and propeller, in the water.

The institute said in a statement: "Contrary to its claims to be a marine wildlife conservation group, in reality [Sea Shepherd] are dedicated to fundraising and to spread violence under pretext of protecting whales.

"[Its] obstructionist activities threaten the lives and property of those involved in our research, are very dangerous and cannot be forgiven."

Glenn Inwood, the institute's spokesman in New Zealand, said the whalers' footage of the incident disproved the activists' account.

"Sea Shepherd's claim that the Shonan Maru has rammed the Ady Gil and cut it in half is just not vindicated by the video," he added.

Sea Shepherd had hoped to use the Ady Gil to frustrate the whalers and force them to abandon the hunt.

It will now be left to the Steve Irwin and the Bob Barker, a Norwegian Antarctic harpoon vessel, to continue the pursuit.

The standoff began earlier today when activists aboard the Ady Gil threw stinkbombs, made from rancid butter, on to the deck of the whaling fleet's mother ship, the Nisshin Maru.

Watson said the loss of the Ady Gil would not deter Sea Shepherd from pursuing the fleet. The group claims it has made significant dents in the whaling fleet's cull in recent years. Last year, it said the fleet returned to port with about 350 fewer whales than planned.

Australia and New Zealand, the closest countries to the scene of the confrontation, today urged both sides to show restraint.

"Our strongest condemnation applies to any violent or dangerous activity that takes place in these remote and inhospitable waters," said the Australian environment minister, Peter Garrett.